Our understanding of the basic mechanisms underlying the initiation, establishment, and metastasis of prostate-cancer is extremely limited. This is due tin part to the complex, heterogeneous, and multifocal nature of prostate cancer. Substantial evidence is accumulating the cancer is a genetic disease of the cell cycle. Two classes of mutations impinge directly on the basic cell cycle machinery that controls cell proliferation; 1) those that activate positive growth pathways, such as overexpression of G1 cyclins, and 2) those that inactivate negative growth control pathways. Negative growth control is mediated by tumor suppressor proteins such as Rb and p53 as well as through the action of two newly defined protein families exemplified by p21CIP1 and p16MTS1. These are inhibitors of cyclin-dependent kinases (Cyclin-Kinase Inhibitors, CK1s) which drive cell proliferation. We have recently fond that p21 family members are expressed primarily in differentiated cells during development (including prostate epithelium), suggesting that they function to mediate cell cycle arrest during terminal differentiation in particular cell lineages. These proteins, in conjunction with other negative regulators, serve to block cell proliferation in adult =tissues, and their activities must be overcome in order to achieve the transformed state. While Rb and p53 are found mutated in a subset of prostate cancers, how other negative growth control pathways are disrupted in prostate cancer remains an open question, although we have found that loss of p21 expression in glandular epithelium is a common feature of prostate cancer. In this proposal, we seek to answer two broad questions with direct relevance to prostate cancer and treatment: 1) which CK1s altered in prostate cancer and metastasis. The specific aims are: 1) to identify the sites and times of expression of CK1s in normal and prostatic tumors. 2) to identify mechanisms which disrupt cell cycle control during prostate tumor development and metastasis, and to examine how potential chemotherapeutic agents such s retinoids block cell cycle progression (3) to identify genes which function to regulate p21 expression in prostate tissue, and 4) to identify new negative cell cycle regulators functioning in the prostate. T his information is critical to our understanding of the molecular mechanisms underlying loss of growth control in prostate. Translational components of this work include th identification of new clinical markers for disease progression, malignancy, and metastasis, the identification of appropriate negative cell cycle control molecules for gene replacement therapy, the identification of mechanisms responsible for the antiproliferative effects of chemotherapeutics including retinoids, and the identification of new genes which regulate terminal differentiation and cell cycle arrest in the prostate.